Fixed point-to-point and broadcast services in C band (4-6 GHz ) and Ku band (11-18 GHz) are expected to comprise the bulk of satellite communications traffic for the foreseeable future. Recently however, commercial satellite systems for use in the Ka-band spectrum (20-30 GHz) have been proposed. Conventionally, payload architectures have been tailored to specfific customer requirements, the satellite operator defining any specific flexibility that is required in terms of being able to select connectivity of specific channels from coverage areas. Once this flexibility has been specified, it is not possible to readily adjust this without affecting the design, schedule and cost of the satellite. This means specific equipment is, to some degree, limited to certain applications and it is not cost effective to optimise them for other applications. The operational lifetime of a communications satellite is typically around about 15 years, which presents a major constraint to operators if the performance of their satellite cannot be adjusted to meet evolving market demand over this period of time. Strong competition from terrestrial service providers means that satellite operators are now demanding more flexible solutions that would enable a satellite's resources to be matched throughout its lifetime to an evolving market requirement.
Most commercial communications satellites operate within the geo-stationary orbit and great care must be taken in designing the satellite's frequency plan and allocating orbital positions to ensure minimal interference between different users. Satellite transponders typically receive signals within one frequency band, process these signals and the retransmit them back to earth within another frequency band. Only certain bands of radio frequency (RF) can be used, with different sub-bands being allocated for signals to and from the satellite. These sub-bands are further divided into channels, with each channel being separated in frequency and having a typical bandwidth of a few 10 s of MHz.
The basic function of a satellite communications transponder is shown in FIG. 1. A weak received signal is passed from antenna 1 to low noise amplifier 2. The channel of interest is selected by filter 3 and then shifted to the appropriate downlink band by down-converter 4. Amplifier 5 provides sufficient gain for it to be transmitted to the Earth via antenna 6. This single conversion process is the most economic in terms of hardware required and is suitable for applications where there are only a few routing combinations between uplink and downlink frequencies.
Many operators now require a single satellite to handle uplink traffic in many bands and direct it to many different downlink bands. In this case, it is common for the transponders to employ double frequency conversion with the channel filtering and routing being performed at an intermediate frequency (IF). The selection and translation of the various frequencies is specified in a frequency plan and FIG. 2 shows a typical example. Some plans can be simple, but as operators replace ageing satellites and consolidate their services on single high performance satellites, frequency plans are becoming more complex. As these plans become more complex, it becomes increasingly difficult to select an appropriate IF such that the local oscillator harmonics and spurious mixing products from each set of conversions do not translate as interference in wanted bands.
Satellite power and mass are at a premium and conventional payload architectures aim to minimise the amount of equipment required to meet the particular frequency plan. A typical payload architecture is shown in FIG. 3, with the filtering and routing functionality embodied in a fixed switching network. Increasing the switch network complexity can confer a limited degree of flexibility in terms of which uplink signals are routed to a particular downlink but this rapidly becomes uneconomic in most applications.
Recent developments in digital signal processing technology have made it feasible to perform all the channel filtering and signal routing within a digital processor. However in space applications, limitations on power consumption and analogue to digital conversion speed has meant that only signals with input bandwidth of up to 100 MHz can be processed. This has proven adequate for high performance mobile communications via satellite and high security military systems.
However in order to manipulate the higher bandwidth signals currently contemplated, several digital processing chains may need to be connected in parallel, with pre-processing of the input signal being performed by analogue means so that each digital processing chain only receives up to 100 MHz.
A functional block diagram representing a digitally processed satellite payload is shown in FIG. 4. In a pre-processing stage, portions of the uplink RF spectrum are frequency converted and conditioned to baseband frequencies that are digitized these for the digital signal processing stage. In the post-processing stage, the processed digital signals are converted into analogue signals, frequency converted and routed to the appropriate downlink antenna path. These pre and post processing stages have very demanding requirements in terms of channel filtering and phase tracking, which are primarily driven by the need to achieve digital beam forming and unambiguous frequency domain processing.
Digital signal processors can perform very precise signal manipulations such as demodulation or very narrow-band filtering (e.g. selecting individual 5 kHz voice channels within a broad band). As is illustrated in FIG. 5, the signals of interest are filtered from the main uplink bands, down-converted in frequency and presented to processor at base-band (DC to about 140 MHz). For example, a single telephone call may arrive at the processor from a C-band uplink originating in one country and be directed to a downlink band that serves a different country. Many different phone calls, originating in different countries, would be grouped in frequency for onward transmission to the common destination country. The processor then sorts the channels within this input band and presents them at the output in the appropriate frequency block, ready for up-conversion to the chosen downlink frequency.
If the signals of interest are relatively wideband, such as TV transmissions, then digital processing may not be economic. In such cases the necessary selection and routing of signals is commonly performed by filters and switches that can operate efficiently at a relatively low intermediate frequency (IF processing).
One method of achieving the desired level of flexibility is described in U.S. Pat. No. 4,228,401, where reconfigurable beam interconnections are facilitated through the use of bandpass filters, each having variable bandwidth and variable center-frequencies. The filters are arranged in groups, each filter within the group passing a selected portion of the frequency band of the received signal. Within each filter, two successive frequency translations of the signal are performed and since the bandwidth and center-frequency of the filter function can be varied, the requirements of that particular channel at that particular time. A similar variable bandwidth filtering and frequency conversion system is described in shown in U.S. Pat. No. 4,262,361
Although, such filters are inherently suited for use in the pre- and post-processing stages of digitally processed satellite payload described above, the complexity of implementation has precluded their use in very high frequency applications such as communications satellites. Instead, Surface Acoustic Wave (SAW) filters have been used, and while these offer excellent channel filtering, they are a major cost driver due to their inherently high phase delay and as such they dominate the overall phase tracking performance.